Many present industries and fields of research are encountering a need for faster and more accurate position and movement determination. For example, in semiconductor fabrication and disk drive assembly the capacity of the ultimate end product depends highly on the accuracy of the measurement systems used, while the economy of the product often depends highly on the speed of the measurement systems used.
Many different measurement systems exist and are in wide use today. Of present interest are optical measurement systems, since they often permit non-contact measurement and have many other desirable characteristics. Present optical systems range from simple triangulation systems which use light beam reflection and geometric principles known since ancient times, to complex laser systems which use interferometric principles to achieve accuracy to within fractions of one light wavelength. However, particularly as modem applications become increasingly complex, the often conflicting goals of measurement accuracy and manufacturing speed remain ones where many seek further improvement.
FIG. 1 (background art) stylistically depicts a measurement system 10 for determining positional information about a movement stage 12. As FIG. 1 illustrates with linear and circular arrowed lines, the movement stage 12 can have its position defined with respect to numerous coordinate systems. For example, positional information about the movement stage 12 can be with respect to each of x, y, and z linear axes, as well as with respect to each of rotational axes for pitch, yaw, and roll. The movement stage 12 thus can be viewed as having as many as has six degrees of freedom. Of course, and as often is the case, movement may be limited to only some of or may not be of interest in only some of these degrees of freedom, but accurate and fast measurement is still often a daunting task.
FIG. 1 includes a first detector 14, a second detector 16, a third detector 18, a controller 20, and an external system 22. The first detector 14 can detect position relative to the x-axis, and provide positional information with respect to this to the controller 20. The second detector 16 can detect position or displacement relative to the y-axis, and provide further positional information about this to the controller 20. The third detector 18 can detect position or displacement relative to the z-axis and provide information about this to the controller 20.
Practitioners of the optical measurement arts will recall that many common detectors today are only able to detect positional change. For example, interferometers can only detect target displacement, a relative position measurement, and not initial or absolute position. Further, if displacement occurs too slow or too fast even these techniques will fail. Herein we generally discuss absolute and relative measurement techniques collectively unless particular differences are important.
Returning to FIG. 1, the controller 20 there provides the positional information it receives, perhaps after appropriate processing and format conversion, to the external system 22. The external system 22 may simply be a display unit that a human user reads, or it may be a servo feedback system precisely controlling various movements of the movement stage 12 in a complex manufacturing process. The external system 22 is thus “external” with respect to the measurement process used; it is merely a recipient of and an ultimate user of the results of the measurement system for some higher purpose.
Unfortunately, the simple position determining system of FIG. 1 can only provide positional information about three degrees of freedom for the movement stage 12. It cannot, for example, tell us anything about roll as depicted by the rotational arrowed line 24. Using detectors of the sort depicted here, adding roll detection would require adding at least a fourth detector 26 (depicted in ghost form) in parallel with the second detector 16. Doing this would thus entail the expenses of more detector hardware, increased controller capability to handle the additional burden of this, and the attendant set-up and maintenance of the more complex position determining system which would result. If the detectors which are used are laser interferometers, as might very well be the case today in a manufacturing or laboratory scenario where high accuracy is necessary, the expense of another detector could be quite appreciable. The costs of additional set-up and maintenance would also likely be appreciable. However, and worth noting for later in this discussion, the added cost for increased controller capability might be quite negligible.
Accordingly, what is needed is a position determining system which employs relatively simple detection hardware yet provides positional information for a measurement target with respect to multiple axes.